1. Field of the Invention
The invention relates to a catalyst degradation detection apparatus and a catalyst degradation detection method.
2. Description of Related Art
In an internal combustion engine mounted in a vehicle such as an automobile, a catalyst for purifying exhaust gas is provided in an exhaust passage, and NOx, HC, and CO in the exhaust gas that flows through the exhaust passage is purified by this catalyst. Also, in order to effectively purify these three components in the exhaust gas, the catalyst is provided with an oxygen storage function and stoichiometric air-fuel ratio control that controls the air-fuel ratio of the air-fuel mixture in the combustion chamber of the internal combustion engine to the stoichiometric air-fuel ratio is performed.
Here, the oxygen storage function of the catalyst is a function that stores oxygen in the exhaust gas in the catalyst and releases oxygen stored in the catalyst into the exhaust gas from the catalyst, according to the oxygen concentration in the exhaust gas that passes through the catalyst. More specifically, when the oxygen concentration in the exhaust gas is greater than the value when the air-fuel mixture in the combustion chamber is combusted when the air-fuel ratio of that air-fuel mixture is the stoichiometric air-fuel ratio, i.e., when the air-fuel mixture in the combustion chamber is combusted at an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, oxygen in the exhaust gas that passes through the catalyst is stored in the catalyst by the oxygen storage function of the catalyst described above. On the other hand, when the oxygen concentration in the exhaust gas is less than the value when the air-fuel mixture is combusted when the air-fuel ratio of the air-fuel mixture in the combustion chamber is the stoichiometric air-fuel ratio, i.e., when the air-fuel mixture in the combustion chamber is an air-fuel ratio that is richer than the stoichiometric air-fuel ratio, oxygen stored in the catalyst is released from the catalyst into the exhaust gas by the oxygen storage function of the catalyst described above.
Also, in the stoichiometric air-fuel ratio control described above, the fuel injection quantity of the internal combustion engine is adjusted according to the oxygen concentration in the exhaust gas such that the oxygen concentration in the exhaust gas comes to match the value when the air-fuel mixture in the combustion chamber is combusted when the air-fuel ratio of that air-fuel mixture is the stoichiometric air-fuel ratio. This kind of stoichiometric air-fuel ratio control uses a catalyst upstream sensor that is provided upstream of the catalyst in the exhaust passage and outputs a signal based on the oxygen concentration in the exhaust gas, and a catalyst downstream sensor that is provided downstream of the catalyst in the exhaust passage and outputs a signal based on the oxygen concentration in the exhaust gas.
More specifically, the fuel injection quantity of the internal combustion engine is adjusted according to the signal output from the catalyst upstream sensor such that the oxygen concentration in the exhaust gas upstream of the catalyst will come to match the value when the air-fuel mixture in the combustion chamber is combusted when the air-fuel ratio of that air-fuel mixture is the stoichiometric air-fuel ratio. As a result, the air-fuel ratio of the air-fuel mixture in the combustion chamber of the internal combustion engine is controlled to converge on the stoichiometric air-fuel ratio while fluctuating between rich and lean. However, with only the adjustment of the fuel injection quantity according to the signal output from the catalyst upstream sensor, the center of fluctuation of the air-fuel ratio of the internal combustion engine that fluctuates between rich and lean to converge on the stoichiometric as described above may be off from the stoichiometric air-fuel ratio due to product variation of the sensor or the like. In order to correct this offset, the fuel injection quantity of the internal combustion engine is also adjusted according to the signal output from the catalyst downstream sensor, such that the center of fluctuation of the air-fuel ratio of the internal combustion engine that fluctuates between rich and lean comes to match the stoichiometric air-fuel ratio as a result of adjusting the fuel injection quantity according to the signal from the catalyst upstream sensor.
In this way, giving the catalyst an oxygen storage function and performing stoichiometric air-fuel ratio control makes it possible to effectively purify the three components in the exhaust gas, i.e., NOx, HC, and CO. More specifically, if the air-fuel ratio of the air-fuel mixture in the combustion chamber changes and becomes lean while stoichiometric air-fuel ratio control is being executed, the oxygen concentration in the exhaust gas that passes through the catalyst will become a larger value than the value when the air-fuel mixture in the combustion chamber is combusted when the air-fuel ratio of that air-fuel mixture is the stoichiometric air-fuel ratio, so oxygen in the exhaust gas that passes through the catalyst will be stored in the catalyst such that the NOx in the exhaust gas is reduced. On the other hand, if the air-fuel ratio of the air-fuel mixture in the combustion chamber changes and becomes rich while stoichiometric air-fuel ratio control is being executed, the oxygen concentration in the exhaust gas that passes through the catalyst will become a smaller value than the value when the air-fuel mixture in the combustion chamber is combusted when the air-fuel ratio of that air-fuel mixture is the stoichiometric air-fuel ratio, so oxygen stored in the catalyst is released from the catalyst, thereby oxidizing the HC and CO in the exhaust gas. As a result, when the air-fuel ratio of the air-fuel mixture in the combustion chamber fluctuates between rich and lean in the process of converging on the stoichiometric air-fuel ratio while stoichiometric air-fuel ratio control is being executed, the three components in the exhaust gas, i.e., NOx, HC, and CO, can be effectively purified as described above.
However, the oxygen storage function of the catalyst decreases as the catalyst degrades, so it is possible to obtain the maximum value of the oxygen storage amount of the catalyst and determine whether the catalyst is degraded based on this maximum value of the oxygen storage amount. This determination as to whether the catalyst is degraded is made according to the procedure below that is described in Japanese Patent Application Publication No. 2008-31901 (JP-A-2008-31901).
Active air-fuel ratio control such as that described below is executed based on a preset condition each time the maximum value of the oxygen storage amount of the catalyst is obtained. In this active air-fuel ratio control, the air-fuel ratio of the internal combustion engine reverses between rich and lean each time there is a reversal of a signal from a catalyst downstream sensor between the lean side and the rich side with respect to a value corresponding to the oxygen concentration in the exhaust gas when the fuel is combusted at the stoichiometric air-fuel ratio, after the air-fuel ratio of the internal combustion engine is forced rich or lean. In this active air-fuel ratio control, the amount of oxygen stored in the catalyst, or the amount of oxygen released from the catalyst, during a preset short time Δt is calculated every short time Δt, during the period of time from after the air-fuel ratio of the internal combustion engine reverses between rich and lean until there is a reversal between the rich side and the lean side of the signal from the catalyst downstream sensor. The amount of oxygen is accumulated each time the amount of oxygen (hereinafter referred to as the “oxygen amount ΔOSA”) is calculated, i.e., every short time Δt, to determine an oxygen storage amount OSA that is indicative of the oxygen storage capacity of the catalyst.
It should be noted that if the air-fuel ratio of the internal combustion engine is changing from rich to lean as a result of the active air-fuel ratio control, oxygen will be stored in the catalyst during the period described above, so the amount of oxygen stored in the catalyst (i.e., oxygen amount ΔOSA) is calculated every short time Δt during this period. The cumulative value of the oxygen amount each times the oxygen amount ΔOSA is calculated serves as the oxygen storage amount OSA. On the other hand, if the air-fuel ratio of the internal combustion engine is changing from lean to rich as a result of the active air-fuel ratio control, oxygen will be released from the catalyst during the period described above, so the amount of oxygen released from the catalyst (i.e., oxygen amount ΔOSA) is calculated every short time Δt during this period. The cumulative value of the oxygen amount each times this oxygen amount ΔOSA is calculated serves as the oxygen storage amount OSA.
In the active air-fuel ratio control, when the signal of the catalyst downstream sensor reverses between the lean side and the rich side after the air-fuel ratio of the internal combustion engine is forced rich or lean, the oxygen storage amount OSA calculated at this time is the maximum value of the catalyst storage amount of the catalyst. Therefore, the determination as to whether the catalyst is degraded can be made based on the oxygen storage amount OSA calculated at the time that the signal of the catalyst downstream sensor reverses between the lean side and the rich side. Also, if the oxygen storage amount OSA is equal to or greater than a preset determining value, it is determined that the catalyst is not degraded (i.e., is normal), but if the oxygen storage amount OSA is less than the preset determining value, it is determined that the catalyst is degraded.
However, the oxygen storage amount OSA used in the determination of whether the catalyst is degraded is a value that includes error due to a response delay time Tr that occurs in the signal of the catalyst downstream sensor, so the oxygen storage amount OSA is a value that is off from the true value of the maximum value of the oxygen storage amount of the catalyst by the amount of this error. The response delay time Tr of the signal of the catalyst downstream sensor is the time that it takes from the time that the oxygen concentration of the exhaust gas around the catalyst downstream sensor changes, for the signal of the sensor to change to a value corresponding to the oxygen concentration after that change. If the oxygen storage amount OSA used in the determination of whether the catalyst is degraded is a value that is off from the true value of the maximum value of the oxygen storage amount of the catalyst, the determination of whether the catalyst is degraded that is based on that oxygen storage amount OSA may be inaccurate.
To deal with this problem, it is possible to use a method for determining whether a catalyst is degraded such as that described below (hereinafter referred to as a “buffer method”). That is, the oxygen storage amount OSA calculated every short time Δt in the active air-fuel ratio control is stored each time it is calculated. The oxygen storage amount OSA that is calculated every short time Δt in this way is a value that is accumulated in increments of the oxygen amount ΔOSA every short time Δt, so the oxygen storage amount OSA increases as shown in FIG. 14, for example, over time. Then, when determining whether the catalyst is degraded (at timing B in the drawing), i.e., when the signal of the catalyst downstream sensor reverses between the rich side and the lean side, the oxygen storage amount OSA stored at a point (i.e., timing A) the response delay time Tr of the catalyst downstream sensor before the point that this determination is made (timing B) is used as the oxygen storage amount OSA for determining whether the catalyst is degraded.
In this case, the oxygen storage amount OSA used to determine whether the catalyst is degraded no longer includes error due to the response delay time Tr of the catalyst downstream sensor. Therefore, the oxygen storage amount OSA will not be a value that is off from the true value of the maximum value of the oxygen storage amount of the catalyst by the amount of the error, so the determination of whether the catalyst is degraded that is based on the oxygen storage amount OSA will not be inaccurate. If the buffer method described above is used, the oxygen storage amount OSA (i.e., the value at timing A in the drawing) that is used to determine whether the catalyst is degraded is able to correspond to the true value of the maximum value of the oxygen storage amount of the catalyst, even if the oxygen storage amount OSA shifts in a non-linear manner as shown in FIG. 15, for example, due to the accumulation over time of the oxygen amount ΔOSA calculated every short time Δt. Thus, the determination of whether the catalyst is degraded is made based on this oxygen storage amount OSA, so the result of this determination is able to be accurate.
Using the buffer method described above as a method for determining whether the catalyst is degraded does make it possible to prevent error due to the response delay time Tr of the signal from the catalyst downstream sensor being included in the oxygen storage amount OSA used to determine whether the catalyst is degraded. More specifically, regardless of whether the oxygen storage amount OSA shifts in a linear manner or a non-linear manner over time due to the accumulation of the oxygen amount ΔOSA calculated every short time Δt, it is possible to prevent the oxygen storage amount OSA used to determine whether the catalyst is degraded from being off from the true value of the maximum value of the oxygen storage amount of the catalyst by the amount of the error.
However, in the buffer method described above, each oxygen storage amount OSA calculated every short time Δt must be stored separately in memory or the like. Also, in order to obtain the effect described above, of the past oxygen storage amounts OSA calculated every short time Δt, the oxygen storage amount OSA for at least the number of calculations more than a value obtained by dividing the response delay time Tr by the short time Δt, and for the most recent of those calculations from the current point, must be stored separately in memory or the like. Therefore, the necessary storage capacity of the memory or the like for storing the oxygen storage amount OSA calculated every short time Δt increases.